The invention relates to a lightning striking distance based on the shielding failure of the power transmission line of the electro-geometric mode and a determining method thereof, and belongs to the lightningproof technical field of high-voltage transmission lines. The lightning striking distance D sv (I, h, V) provided by the invention is described as follows: D sv (I, h, V) is equal to (-0.004I <2> added with 0.221) h <0.625> <e> <Bv>; wherein, I is lightning current, and the unit thereof is KA; h is the height of a horizontal lead; V is the voltage of the horizontal lead. The determining method of the lightning striking distance includes that the quadratic polynomial of each height of the horizontal lead can be estimated; and the equation of the lightning striking distance D (I, h) related to the height of the horizontal lead can be obtained; when the voltage of the horizontal lead is between minor 1MV and plus 1MV, and when the lightning current is within 200Ka, the correction factor of lightning striking distances under different voltages, kv is equal to e <Bv>, can be achieved by the analog simulation, and finally, the lightning striking distance D sv (I, h, V) can be obtained. The lighting striking distance determined by the method provided by the invention can well meet the physical reality and can reduce the jumping rate of the lightning striking of the power transmission line, thereby guaranteeing the safe operation of power transmission lines.